Mutation - bulkUpdateActionLabels

Sets, adds, or removes labels on specified actions. SET replaces the complete label list; ADD and REMOVE update it incrementally. Use SET with an empty list or ["none"] to clear all labels. Requires access to every target action. SET and ADD fail with "Labels do not exist or were deleted: " when a label id was never a label of the workspace, and nothing is written. Deleted labels are not an error: they are left out of what is written, so a SET built from stale or stored labels still saves. REMOVE accepts deleted label ids, so a stale one can be cleared.